1. Field of the Invention
The field of this invention relates to devices for analyzing solids and more particularly to a device for analyzing a solid which uses positrons and the transmitting of those positrons onto the solid that is to be analyzed to result in the obtaining of information about the solid.
2. Description of the Related Art
A positron is an elementary particle with a mass equal to that of an electron and a positive charge equal in magnitude to the negative charge of an electron. The positron is thus the anti-particle of the electron. The positron has the same spin and statistics as the electron. A positron, is, in itself, stable but cannot exist indefinitely in the presence of matter, for it will ultimately collide with an electron. The positron and the electron will be annihilated as a result of the collision, and photons (gamma rays) will be created.
When a positron is injected into a solid substance, the positron will annihilate an electron and release gamma rays. These gamma rays are easily detected and can be used to measure properties of the solid material. Particles which can be released from the solid can also be used to measure properties of the solid material. These phenomena are the basis of a wide variety of techniques that can be used as tools for materials and surface analysis and can provide information that is not available from any other technique. Implementation of these techniques requires high quality positron beams. Currently, these techniques are being used at university laboratories, where the required positron beam systems are constructed by the users. However, such systems have the potential to be employed for industrial applications, such as quality control on integrated circuit production lines. It is this need that the current inventor is seeking to address by designing the subject matter of the present invention.
An important application of positron beams is a wide variety of techniques that have been developed for the analysis of solids and surfaces. Each of these techniques has its own characteristic positron beam requirements. Some employ steady state beams while others require pulsed beams. Several will provide usable data with relatively large diameter beams that are typical of radioactive positron source diameters (several millimeters) while other are applicable only using microbeams (less than ten microns in diameter).
In addition to surface analysis techniques, positrons can also be used to analyze properties of solids below the surface of the solid. This unique feature of positron based techniques arises because it is possible to measure the annihilation gamma rays from high energy positrons, which can easily penetrate to the surface of the solid. By varying the energy of the incident positron over a range of a few kilo-electron volts to greater than one hundred kilo-electron volts, positrons can be implanted to varying depths, thus permitting depth profiling of the properties of the solid. The information is contained either in the lifetime of the positrons or in the shape of the gamma ray line, which is Doppler-broadened by the momentum of the annihilated electrons and thus provides information about the chemical environment of the annihilation site.
The following is a list of some of the current applications for positron beams:
1. Positron Remission Spectroscopy (PRS)xe2x80x94This technique is based on the phenomenon that positrons implanted near the surface of a solid can thermalize, that is come to the same temperature as the surface of the solid, and be reemitted. The energy of the reemitted positrons can be analyzed to yield information that is not available with conventional scanning electron microscopy. This PRS technique has the ability to distinguish non-uniform film thickness, varying crystal orientation, differences in concentrations of microscopic voids in the crystal structure, concentrations of adsorbed molecules and contaminant layers.
2. Positron Annihilation Induced Auger Electron Spectroscopy (PAES)xe2x80x94This technique is analogous to Electron Induced Auger Electron Spectroscopy (AES), except that the core hole, which leads to the ejection of the Auger electron, is created by positron annihilation rather than electron impact. For this technique, positrons are injected at low energy into the surface to be analyzed. The ejected electrons are analyzed in the usual way using an electron energy spectrometer, but the measurement is substantially simplified because of the absence of background high-energy secondary electrons.
3. Reemitted Positron Energy Loss Spectroscopy (REPELS)xe2x80x94In this process, low energy monoenergetic positrons bombard the surface to be studied, and those that are reflected are energy analyzed. Energy is lost by transfer to vibrational modes and electronic state transitions of the surface and surface absorbed molecules. By measuring the magnitude of the energy lost, information can be obtained about the chemical composition of the surface of the solid and of surface absorbed molecules.
4. Low-energy Positron Diffraction (LEPD)xe2x80x94For this technique, a crystalline sample is bombarded with low energy (0-300 electron volts) monoenergetic positrons. Because of the low energy, there is relatively little penetration into the solid, and some of the positrons are reflected producing spots on a phosphor screen. This information can be used to determine the crystal structure of a clean substrate or to analyze an adsorbed layer.
5. Positron Induced Ion Desorption Spectroscopy (PIIDS)xe2x80x94This relatively new technique uses positrons to eject ions from the surface of the solid and measures the time required for the ions to reach a detector. Hence, the mass of the ions can be determined. The rate at which the ions are ejected from the surface of the solid is much greater when positrons are used rather than photons.
6. Positron Annihilation Lifetime Spectroscopy (PALS)xe2x80x94Positrons injected into a surface can accumulate in microscopic voids of the solid where such will eventually annihilate an electron of the solid. For high energy positrons obtained directly from the radioisotope of sodium, sodium-22, the lifetime can be measured by recording the time delay between the 1.2 million electron volt gamma rays that are emitted by the sodium nucleus simultaneously with the positron and the 511 kilo-electron volt annihilation gamma rays. This technique has been extensively applied to the study of bulk properties of solids. One of the most important current applications of lifetime spectroscopy is the analysis of microvoids in semiconductors and polymers. This technique is the most sensitive one available for studying voids in solids and can provide information about both the size and concentration of the voids. The technique has been applied to characterizing the properties of semiconductors, such as ion-implanted silicon, to study, for example, stress voiding and electromigration. One of the most important current areas of research is the study of the properties of polymers. Positron lifetime spectroscopy is capable of measuring the size and distribution of voids, which determine properties such as strength of the solid, gas permeability of the solid and aging characteristics of the solid. Another important topic is the development of insulators with low dielectric constant for use in microelectronic fabrication. Such insulators are essential for increasing microprocessor speeds and the positron technique described can be used to measure the properties of these insulators.
7. Variable Energy Positron Lifetime Spectroscopy (VEPLS)xe2x80x94The power of the PALS technique can be substantially enhanced by implementing it using a positron beam source of constant energy rather than a radioactive source which has a range of energies. By varying the beam energy, positrons can be implanted to varying depths so that a depth profile of void size and concentration can be obtained. Furthermore, if the beam diameter is small, it can be scanned across the surface of the solid so that three dimensional information can be obtained. This three dimensional information is obtained by scanning the positron beam across the surface when the size of the beam is smaller than the characteristic features of the solid being examined, such as a transistor. This technique requires positron pulses that are shorter than the typical time it takes a positron to annihilate an electron in a solid.
8. Positron Annihilation Spectroscopy (PAS)xe2x80x94This technique measures the Doppler-broadening of the 511 kilo-electron volt gamma ray line resulting from the annihilation of positrons implanted into solids. The required information is contained in the gamma ray line shape. PAS can provide the same type of information about defects as PALS and VEPLS.
Of the techniques numerated above, PALS, PAS and VEPLS have the capability of providing information to the integrated circuit manufacturing industry that is not available from any other technique. All of the above techniques can be used to obtain information about surfaces and solids in two and three dimensions if such are applied using beams of small diameter (10 microns or less). Such beams can be scanned across a surface and the emitted particles or gamma rays analyzed at each position to create two-dimensional images of surface properties. Furthermore, if this procedure is repeated using positrons of different energies, three dimension images can be created that reveal sub-surface structures and properties.
Positron microbeams have been created using the technique called remoderation brightness enhancement. This method reduces the beam diameter by a factor of ten but has the unwanted side affect of reducing the beam intensity by seventy percent or more. Microbeams can be created from large diameter beams by repeating this process several times. The flux of the resulting beams are typically only a few percent of the incident beam. This means that long data acquisition times are required to obtain useful data. The present invention has addressed this problem by producing positron microbeams with much greater efficiency than is possible using remoderation brightness enhancement.
A positron beam producing apparatus which utilizes a vacuum chamber with a source of positrons within the vacuum chamber and the positrons forming a positron cloud. The apparatus is used to compress the positron cloud and to produce a thin positron beam which is extracted from the cloud. The positron beam is transmitted to a focusing device and from the focusing device to a target where gamma rays and particles are released which can be used to analyze the properties of the target.
The positron producing apparatus, as previously described, plus use of a cooling gas within the vacuum chamber.
The positron producing apparatus, as previously described, which further includes use of an electrostatic lens as the focusing device.
The positron producing apparatus, as previously described, which further includes a series of annular electrodes located axially through which the positrons are to be passed.
The positron producing apparatus, as previously described, which further includes use of a rotating electric field within the positron cloud compressing device.
The positron producing apparatus, as previously described, which further includes a series of Penning Traps as the positron cloud compressing device.
A method of compressing a positron cloud producing a thin positron beam which is to be transmitted onto the solid for the purpose of analyzing properties of the solid which comprises the steps of supplying a source of positrons within a vacuum environment, forming a positron cloud, subjecting the positron cloud to magnetic and electric fields to confine the positrons, subjecting the positron cloud to a rotating electric field to compress the cloud, extracting a positron beam from the cloud, focusing of the positron beam, and transmitting of the focused beam onto the solid.
A further method of compressing a positron cloud where the previously described method also adds a cooling gas into the vacuum environment.
The positron beam producing apparatus of the present invention is a versatile, high quality, positron beam source that can supply positrons for many of the analytical tools that have been used in the past and have been described in the Background of the Invention. The positron beam producing apparatus of the present invention is to provide commercial availability of a low cost, compact, user friendly positron beam source that is expected to be useful to a variety of users worldwide.